This invention relates to a light-emitting device provided with a light-emitting layer structure by utilizing electroluminescence of a light-emitting material which emits light by passing an electric current therethrough under an applied electric field and which is formed in a layered structure, and, in particular, it relates to an organic light-emitting diode in which the light-emitting layer structure is constructed from a hetero-PN-junction comprising an organic semiconductor as a light-emitting zone.
An electroluminescent device (hereinafter, abbreviated to xe2x80x9cEL devicexe2x80x9d) is a device which emits light by applying an electric field to a substance, especially a semiconductor. Light-emitting diodes are a well-known example of EL devices made from inorganic compound semiconductors of an element (elements) of: Groups III to V of the Periodic Table such as GaAs or GaP. These devices emit light effectively at a long wavelength side of a visible light region and are widely utilized for daily electronic appliances, but they have their limit in size and therefore the easy and economical utilization thereof in a large-area display has not yet been realized. As an alternative structure capable of being produced over a large area, thin film type EL devices are well-known which are produced from an inorganic material by doping semiconductors of compounds derived from an element or elements of the Groups II to IV of the Periodic Table such as ZnS, CaS and SrS with Mn or a rare earth metal, for example, Eu, Ce, Tb or Sm as a light-emitting center. However, for driving the EL devices using these inorganic semiconductors, an alternating current electricity and a high voltage are required and thus such EL devices are expensive, and further a full-color device is difficult to obtain.
In order to solve the above problems, EL devices using an organic thin film is extensively studied. For example, such EL devices include those using a vapor deposition film of an organic luminescent (fluorescent) dye reported in the following literature references:
(1) S. Hayashi et al., J. Appl. Phys. 25, L773 (1986)
(2) C. W. Tang et al., Appl. Phys. Lett., 51, 913 (1987)
To date, organic EL devices which emit light of from blue to red colors have been developed. Details of the organic electroluminescence are described in, for example, the following literature references:
(1) xe2x80x9cOrganic EL Device Development Strategyxe2x80x9d, compiled by Next Generation Display Device Research Association, Science Forum (published 1992)
(2) xe2x80x9cElectroluminescent Materials, Devices, and Large-Screen Displaysxe2x80x9d, SPIE Proceedings Vol. 1910 (1993), E. M. Conwell and M. R. Miller
Further, in recent years, a thin film is easily formed by a spin-coating, and an EL device using a thermally stable poly (arylene vinylene) polymer has been studied. Such EL devices using the poly (arylene vinylene) polymer are described in, for example, the following literature references:
(1) J. H. Burroughes, Nature, 347, 539 (1990)
(2) D. Braun, Appl. Phys. Lett., 58, 1982 (1991)
These organic EL devices emit a light, upon injection of an electric current, by recombination of the electron and the injected hole in the process of a radiative decay of the produced excitons. All organic EL devices known to date are based on the following three kinds of typical two- or multi-layer device structures, that is, 1) as shown in FIG. 1, an organic EL device having a two-layer structure comprising an organic light-emitting layer 2 and a hole transport layer 3 sandwiched in between a metal cathode 1 and a transparent anode 4 on a transparent substrate 5, which device emits light when the hole from the hole transport layer is injected into the light-emitting layer where the hole is combined with the electron; 2) as shown in FIG. 1, an organic EL device having a two-layer structure comprising an electron transport layer and an organic light-emitting layer, which device emits light when the electron from the electron transport layer is injected into the light-emitting layer where the hole is combined with the electron; and 3) as shown in FIG. 2, an organic EL device having a three-layer structure comprising an electron transport layer 6, an organic light-emitting layer 2 and a hole transport layer 3, sandwiched in between an metal cathode 1 and a transparent anode 4 on a transparent substrate 5, which device emits light when both the electron from the electron transport layer and the hole from the hole transport layer are injected into the light-emitting layer where the hole is combined with the electron.
However, in the conventional organic EL devices in which two- or multi-layer organic thin films are used, only one layer emits light while the other layers concerned with electron or hole transport. In addition, the conventional organic EL devices still have unsolved technical problems in luminous efficiency and device lifetime. As a result of extensive studies in order to develop an EL device having unique and excellent characteristics, the present inventors found that an organic pn-junction can be appropriately prepared by consideration of ionization potential, electron energy bandgap, Fermi level and mobility of an p-type organic luminescent-semiconductor material and an n-type organic luminescent-semiconductor material as well as thickness of these organic thin films, and that an organic light-emitting diode using the organic pn-junction can be obtained, and completed the present invention based on the above discovery.
An object of the present invention is therefore to provide an organic light-emitting diode having high luminance and high luminous efficiency.
The thin film of the organic compounds is a certain type of semiconductors, and, in an organic EL device composed of a layered organic thin films, electric properties on a contacting boundary surface thereof determine the properties of the organic EL device per se. For this reason, the relationship between each level of ionization potential, electron energy bandgap, Fermi level and mobility of the organic thin layers on contacting boundary surfaces, and the work functions of an anode and a cathode is very important. With consideration of the above relationship, the present invention finally reached an organic EL device having high luminous efficiency by using a semiconductor model.
The organic pn-junction is constructed by laminating two adjacent thin organic luminescent-semiconductor films which are different in the bandgap. At the time of thermal equilibrium, the total Fermi level in the two materials is constant. The thermal equilibrium is achieved by diffusion of a free electron carrier through a junction surface, and, as a result, an internal electric field is generated at the junction. Due to this internal electric field, shift of vacuum level and bending of band edge are generated. The internal electric field generated in conduction bands of a p-type semiconductor material and an n-type semiconductor material functions as a voltage barrier which prevents transfer of electrons escaping from the n-type material portion to the p-type material portion. Also, in a similar manner, the internal electric field generated in valence electron band functions as a voltage barrier which prevents transfer of holes from the p-type material portion to the n-type material portion. When forward bias is applied, electrons are injected into the n-type material from the cathode and holes are injected into the p-type material from the anode. Thus, the injected electrons and holes are accumulated on the boundary surface of pn-junction. When a bias voltage exceeds a certain level, the electron enters a p-type zone of the pn-junction over the voltage barrier and is recombined with the hole thereby producing molecular excitons.
A singlet molecular exciton emits luminescence thereby emitting light having a central wavelength determined by the electron energy bandgap of a p-type luminescent material. Also, when a bias voltage exceeds another level, the hole enters an n-type material portion over the voltage barrier and is recombined with the electron thereby producing molecular excitons similarly. A singlet molecular exciton emits luminescence thereby emitting light having a central wavelength determined by the electron energy bandgap of an n-type luminescent-material. On the other hand, when a luminescent material having a small bandgap is excited by the light emitted by a luminescent material having a large bandgap, photoluminescence is generated. All of these emissions are accumulated to form light emitted by the pn-junction. The emission spectrum distribution of the light is obtained by convolution treatment of the spectrum from each emission portion. The ratio of these emission portions is determined by the injection balance of holes and electrons at the pn-junction.
Each of the organic p-type luminescent-semiconductor and the organic n-type luminescent-semiconductor used preferably has bandgap energies ranging from 1 eV to 3.5 eV.
In the present invention, as a result of studies on each of absolute values of the ionization potentials IP1 and IP2 of the organic p-type luminescent material and the organic n-type luminescent material as well as on each of absolute values of the electron affinity X1 and X2 thereof, it was found that, in the pn-junction fabricated by using the organic p-type luminescent material and the organic n-type luminescent material satisfying the relationship of the following equations 1 to 3, electrons and holes are recombined at the same time in both the p-type material zone and the n-type material zone when forward bias is applied, and thus light is emitted from both the p-type luminescent material and the n-type luminescent material:
X1xe2x89xa6X2xe2x80x83xe2x80x83Equation 1:
IP1xe2x89xa6IP2xe2x80x83xe2x80x83Equation 2:
xe2x88x920.2eVxe2x89xa6(IP2xe2x88x92IP1)xe2x88x92(X2xe2x88x92X1)xe2x89xa60.2 eVxe2x80x83xe2x80x83Equation 3:
wherein X1 denotes an absolute value of the electron affinity of the organic p-type luminescent-semiconductor, X2 denotes an absolute value of the electron affinity of the organic n-type luminescent-semiconductor, IP1 denotes an ionization potential of the organic p-type luminescent-semiconductor and IP2 denotes an ionization potential of the organic n-type luminescent-semiconductor.
As described above, when the relative difference in the bandgap energy between the adjacent materials becomes narrow, the organic hetero-pn-junction has a high injection efficiently of electrons and holes and, as a result, the resistance of charge injection through a boundary surface of the junction becomes low whereby the voltage applied can be lowered. Therefore, high light-emission efficiency can be obtained by a low drive voltage. Furthermore, due to decreased resistance on a boundary surface of the junction, Joule""s heat generated during the drive of EL device decreases. As a result, deterioration of the device by heating can be prevented thus to lengthen the life of the device. When the relative difference in the bandgap energy between the adjacent materials is 0, namely, if both of the p-type luminescent-semiconductor and the n-type luminescent-semiconductor are made of the same organic luminescent material having the same bandgap energy, the organic pn-junction is an organic homo-pn-junction. Higher efficiency can be expected from the organic EL device. The present invention was completed by the above-described discovery.